Peptide antigen presentation via major histacompatability complexes has long been recognized as a central element in adaptive immune responses. Recently, a parallel pathway that can elicit potent immune responses has begun to be elucidated. This pathway involves the presentation of glycolipids by CD1 proteins and is believed to be responsible for a portion of the innate immunity of mammals to bacteria.
The CD1 locus of mammals includes five distinct isotypes, CD1a, CD1b, CD1c, CD1d, and CD1e. These nonpolymorphic, membrane-bound proteins are characterized by their ability to present classes of glycolipids to T cells. The CD1d member of the gene family has been characterized by its ability to bind and present α-galactosylceramides to natural killer T cells (NKT cells).
Complex formation between glycolipid-loaded CD1d proteins and T cell receptors can subsequently lead to the stimulation of T cells. T cell stimulation initiates, inter alia, the cellular production of certain key immunoresponsive biochemicals.